Hydraulic trim control



May 11, 1954 T. A. FEENEY ETAI.

HYDRAULIC TRIM CONTROL 4 Sheets-Sheet l Filed Jan. 24. 1949 Rmx T. A. FEENEY ET AL May 11, 1954 i HYDRAULIC TRIM CONTROL Filed Jan. 24. 1949 f5 4a fea/v 25 fascism/N640 4 Sheets-Sheet 2 INVENTORS /o/wms ffNfY BY [06f/vf l( 52am/5 May 11, 1954 T. A. FEENEY ETAL HYDRAULIC TRIM CONTROL Filed Jan. 24, 1949 4 Sheets-Sheet 5 INVENTORS MM45/4. Z'fA/fr May 11,` 1954 T. A. FEENEY ET AL. 2,678,179

HYDRAULIC TRIM CONTROL Filed Jan. 24. 1949 4 Sheets-Sheet 4 C 6 i-* .f2

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@fsm/eroe 10.5 104 .Samva/o rave 106 v79 2 ez e? IN VEN TOR //a/ms A /Zf/vfr BY 065,116 I( Beam/- immun #auf spl/N6 f onze Patented May 11, 1954 UNITED STATES PATENT OFFICE HYDRAULIC TRIM CONTROL fornia Application January 24, 1949, Serial No. 72,412

22 Claims. 1

The present invention relates to airplane control systems, and more particularly to an airplane control system wherein trim `can be achieved by adjustment` ofthe relation between a pilots control column and a restoring force producing mechanism, so that trim can be achieved by the use of the control surfaces themselves, without the application of pilot force.

In high speed airplanes, particularly multimotored pursuit or combattypes, frequently only a single pilot is available to handle the controlsI of the airplane. Due to the ever increasing use of instruments additional to engine condition indicators, such as radar `for example, the trimming of the airplane quicklyand eiiicently becomes a major task for the pilot, particularly `for sudden out-of-trim moments that might take place while the pilots attention is otherwise directed. The usual sequence of pilot response to an out-of-trim moment is, iirst, the use of the normal controls by manual application of control column and rudder pedal forces, and then the operation of various trim tabs until trim `is restored, thus releasing the pilot `from maintaining the control forces.

rthe customary trimming procedure, however, entails operation oi one or more `of the trim control elements for elevation, direction, and lateral trim by the pilot Yas maybe needed, which `distracts the pilots attention from what might well be other important operations. It is, therefore, an object of the present invention to provide a quick and efficient centraliaing andtrim system that will normally centralize the controls, automatically maintain trim and, in addition, will remove control element trim forces from the pilot with only momentary attention by the pilot. Another object of the invention is to provide a trim system utilizing the normal airplane control surfaces which may, if desired, be completely power operated.

When control surfaces are full power actuated, as, for example, by a hydraulic servomotor, the pilot `cannot feel any -of the aerodynamic forces acting on the control surfaces. `In consequence, it is customary when the surfaces are full power operated. to utilize balanced elastic restoring forces appliedto the `control column and rudders so that the control column neutral as determined the applied elastic forces substantially corresponds to the normal operating neutral vposition of the control surface with power 'operated controls, which require very low column forces for operation, and which do not `feed back surface loads to 'the control column. A shift inthe point of application of the restoring forces to the control column will shift the column position and, in consequence, the position of the controlled surfaces. When this shift is made to counteract an out-of-trim moment the airplane will be held in trim position and the control column can then be operated as usual from the new neutral position. If and when the out-of-trim moment disappears, the control column can be restored to its original neutral. It is another object of the present invention to provide a simple and eiiective means and method of changing the control column neutral as desired by yan airplane pilot, in order to trim the airplane.

In a companion application, Serial No. 24,226, led April 30, 1948, now Patent No. 2,6`39,108,`May i9, 1953, Feeney and Hall have shown, described and claimed an automatic trim system for airplane control surfaces wherein an adjustablelink is changed in length to change the relationship between the control column and the restoring force producing mechanism. The length of the link, in the preferred embodiment of the invention shown and described, is changed electrically under the control of the pilot, utilizing electrical power from a source such as a battery or engine driven generator. It is another object oi the present invention to provide a means and method of adjusting the linkage between a restoring force producer and the control column and control surfaces, that does not require an external source of power, and which will, in consequence, be operative at all times.

Our invention can be more fully understood by reference to the drawings, in which one preferred embodiment of the present invention is shown as applied to a stick type control of two surfaces movable in the same direction ror elevation control and in opposite directions for roll control, these surfaces being shown as elevons.

In the drawings:

Figure l is a perspective diagrammatic View partially top sectional, showing a control stick system as connected to operate the elevons of an airplane, the cable connections and operating motors of only one eleven being shown.

Figure 2 is a side view partly in section -and partly diagrammatic, of the elevator and aileron stick motion combining mechanism, showing also the elevator stick-force producer used in the system of Figure l.

Figure 3 is an enlarged topsectional viewshowing the control stick mounting bearings, taken as indicated by broken line 3-3 in Figure 2.

`Figure 4 is an enlarged 4cross-sectional view showing the control stick attachments, taken asY indicated by broken line l-l in Figure 2.

Figure is a longitudinal sectional View of the trim cylinder and piston used in the force producer of Figure 2.

Figure 5A is an end view of the trim cylinder, viewed as indicated by the arrow 5A in Figure 5.

Figure 6 is a schematic diagram showing the hydraulic circuit of the elevator trim system.

Figure '7 is a longitudinal sectional view of the restoring spring, taken as indicated by the line l-l in Figure 2.

Figure 8 is a schematic diagram showing in simplified form another embodiment of the trim system in Figure 2.

Referring first to Figures l and 2, a pair of concentric torque tubes I and 2 are mounted for independent rotation on a fore-and-aft axis in the airplane on tube bearings 3. The inner tube l extends beyond the forward end of the outer tube 2 and carries a laterally extending inner tube gear sector il, and the forward end of outer tube 2 is provided with a similar outer tube gear sector 5 extending laterally opposite 'the tube gear sector li.

The inner tube I is extended forwardly to form a stub shaft of smaller diameter than the inner tube i and a stick bearing i is mounted on the stub shaft E for free rotation thereon. The forward end of stub shaft 6 rotates in a stub shaft bearing t, attached to the airframe. As shown in Figures 3 and 4, a cross shaft 9, normally horizontal, is mounted on stick bearing 'l and shaft end bearings Ill, lil are attached to cross shaft ll by opposite cross shaft pins l l, the axes of which are substantially in a horizontal plane perpendicular to the stub shaft t. A stick yoke i is 4rotatably mounted around the cross shaft s by means of the shaft end bearings it. This yoke l2 also encircles the stub shaft t, with a space i3 provided to permit a rotation of the yoke l2 on shaft bearings I@ of about 45 or more. Thus, the yoke i2 when assembled can rotate in a trans verse vertical plane on stick bearing l and in a fore-andaft vertical plane on shaft bearings l@ either with single or combined rotations.

Opposite sides It and l5 of the yoke i2 near the ends of the cross shaft Q are respectively fixed to an outer tube pinion i6 and an inner tube pinion i7, these pinions meshing respectively with the outer tube gear sector 5 and the inner tube gear sector 4.

The upper part of the yoke i2 around the cross shaft 9 and stub shaft t forms a socket it into which is pressed and locked a pilots control stick E3. A stick handle 28 at the 'top of the control stick i9 is provided with a momentary-contact trim switch button 2 i, as shown in Figure 2.

Thelaft ends of the inner and outer torque tubes i and 2 are respectively connected to elevon cable quadrants 22 and 23 provided with cable grooves 2d in their arcuute surfaces. An elevon up cable and an elevon down cable 26 are each attached at opposite ends of cach quadrant cable groove 24 and, following one pair of cables as shown for the left-hand elevon control, lead over cable pulleys 2l to pass around lever pulleys 23 mounted on the end of a valve control lever Zi?. The elevon up cable 25 passes around an idler pulley t also mounted on control lever 29 and then terminates in a tension regulator 3l mounted on control lever 29. The elevon down cable 2t enters the tension regulator directly from the opposite lever pulley.

Control lever 29 is mounted on a lever shaft 32 having top and bottom valve arms 33, each connected to operate a hydraulic servo valve spool (not shown) in a servo valve casing 3ft attached to a hydraulic actuating cylinder 35, through a spring rod 36 having a two-way spring (not shown) enclosed in a spring casing 3l.

As both hydraulic motors are alike except for being reversed in position, the identical parts of each will be given identical numbers and the connections of only one motor will be described.

An actuating piston rod All attached to a piston (not shown) inside the hydraulic actuating cylinder 35 is rotatably attached to the airframe by a rod bracket 4I to extend substantially parallel to spring rod 36. lIhe hydraulic actuating cylinder 35 is attached by cylinder bracket i2 to a bellcrank 43 mounted to rotate on axis lill, this bellcrank being connected by link i5 extending beyond the airframe to rotatably connect with the elevon (not shown) 'beneath the axis of rotation thereof. It will be noted that the twomotors are connected through the elevon structure. The type of full power control surface operating system described briefly herein is more fully shown, described and claimed in the co-pending applican tion of Feeney, Serial No. 23,567, filed April 27, 1948.

The operation of the controls so far described and without considering the elevator centralizing or trimming system later to be described, is as follows:

The stick i9, for example, when moved aft without lateral motion thereof, rotates both pinions l5 and il in the same direction, and due to the toothl mesh with the respective inner and outer tube gear sectors 4 and 5 will rotate the attached torque tubes in opposite directions to actuate the elevon up cables from both quadrants 22 and '23.

These cables then through control lever 29, valve arms 33 and spring rods 36 will move the spools in each hydraulic valve casing 34 to apply hydraulic fluid under pressure to the proper side of the hydraulic actuating piston to move the actuating cylinders in a direction to move the elevons on both sides of the airplane center line upwardly. As the elevons move, however, they move the actuating cylinder in the same direction as the original spool movement, until the f low to the actuating cylinder is shut oif. A similar action takes place when the stick is moved for ward to actuate the elevon down cables, whereupon the elevon will be moved downwardly. As the actuating cylinders and valves are moved with the surface, and the spool is moved only by the control stick, the surfaces follow all stick movements, move upwardly or downwardly in accordance with the amount of stick movement, but transmit no significant air loads to the stick as the only connection between the surfaces and the control system is by way of the spool friction, which is, of course, negligible as compared even to friction forces in the system.

As two actuating cylinders are directly connected together through the elevon, itis desirable that one valve of each pair of cylinders remain operable if the other valve should jam, consequently, the spring rods 36 are provided so that the rod attached to operate the jammed valve can elongate or shorten to permit operation of the other valve. Normally, spring rods 3S operate as solid rods as far as valve operation is concerned.

Returning to the description of stick operation, when the stickA i9 is rocked laterally, the pinion on one side will be moved downwardly and the pinion on the other side will be movedupwardly, carrying the gear sectors with them, thus rotating `both torque tubes l and 2 together and in the same direction. rlhis rotation will cause the quadrants to move together, thereby operating the up cable to one pair of hydraulic motors, and the down cable to the other pair of hydraulic motors, the result being to move one elevon up and the other elevon down for roll control.

As both elevation and roll control stick motions can take place simultaneously, the corresponding movements of the elevons take place simultaneously and full control in elevation and roll is obtained.

Stick centralizing forces are applied in aileron movements of the stick by a two-way aileron centralizing spring 48 extending from one end of the cross shaft 9 to the airframe. In the system shown, these forces are not adjustable in strength or in application point.

Centering forces are applied to the stick in elevation directions by the use of a trim rod 49 rotatably attached to the lower part of the stick yoke l2 under the stub shaft 6 by trim rod bearing 50 and extending aft to attach to a floating hydraulic trim cylinder 5l whose trim piston rod 52 is also extended further aft to end in a universal motion bearing "53 carried on a spoke 54, this entire combination forming an extensible link 55. The detailed construction and operation of trim cylinder 5| will be described later.

Spoke 54 is attached to a trim shaft 56, mounted on bearings on each side of torque tubes l and 2 with the trim shaft rotational axis crossing the projected rotational axis of torque tubes l and 2 at a right angle.

A cam sector 5l is mounted on one end of trim shaft 56 with a cam surface 58 facing forward, the cam having a central indent portion 59 of minimum radius and side portions of greater radii. Opposite the indent 5@ of the cam, the trim shaft carries a force lever Sil rotatably connected to a spring operating rod di entering an elevator force producer cylinder 62 attached to the airframe by support 63 and held in a substantially vertical plane. The detailed construction of the force producer will be later described.

In front of cam sector 5l a cam lever 6d is rotatably attached below to the airframe, and extends upwardly to rotatably connect to a bellows rod 65 extending forwardly. Cam lever Ell, between its ends, also carries a cam follower 65 rolling on cam surface 53 so that pressure on bellows rod 65 directed aft will force the cam follower 66 into the central indent portion 59 of cam surface 5E; and thus rotate trim shaft 5S if the cam follower is not in the indent portion.

In night, the force on bellows rod c5 is supplied by the dynamic air flow by means of a bellows casing 5l preferably spherical and containing a diaphragm 68 attached to the casing 5l' by ilexible edges E9, this diaphragm dividing the casing into a pressure chamber lll and a static chamber Tl. Bellows rod 65 extends through the casing 6l', and diaphragm 68 is attached at a right angle thereto inside the casing El. A light diaphragm spring 'l2 presses the diaphragm aft to insure contact of follower 5G with cam surface 58 when the airplane is not in flight. Pressure chamber 'lll is connected by ram air tube 13 to an air scoop, for example, so that pressures in accordance with air speed can be produced `in pressure chamber 10. The static chamber 1l is connected by static line 74 to a position of static or negative pressure on the outside ofthe airplane. The bellows action applies a synthetic feel to the control stick when moved in elevation direction. A similar device for producing synthetic feel is shown, described and claimed .in the co-pending application of Ashkenas, Serial No. 52,367, filed October 1, 1948, now abandoned. The device is valuable in airplanes having full powered control surfaces, as the aerodynamic forces are reduced as the airplane approaches a stall. The pilot can feel this reduction and thereby receives a stall warning.

C-ne satisfactory elastic centering force producing mechanism is shown inFigure 7 and will next be described.

Spring operating rod 5l is hollow, and is provided with end shoulder 15 which bears against an internal nut 1%' on one end of a ten sion rod il. Tension rod 11 passes through one spring cap 'I8 and then through a second and opposite spring cap 'i9 to terminate in a second rod nut ile. A compression spring 8l, preferably pro-loaded, is placed between spring caps 'F8 and lil, and caps 'FB and 'lil are movable within the force producer cylinder G2, being held from leaving the cylinder at one end by cylinder end 82 and at the other end by split ring 83. The compression spring 8! is the elevator force producer. Spring operating rod 6l bears against, but is not attached to, spring cap T3.

Spring operating rod 6! acting under the control of compression spring 8| is connected to the control stiel: i9 through force lever El), trim shaft 5S, spoke 54, trim piston rod 52, trim cylinder 5i and trim rod 49, to hold the stick in a predetermined neutral position which normally is the position from which the attached control surfaces are moved for aerodynamic elevation control of the airplane. Thereafter, movement of the control stick either fore or aft will compress compression spring 8| to supply a restoring or centralizing force to the stick, acting as follows, considering that no change in relation of the hydraulic trim cylinder and its piston occurs.

When force lever 5l! is moved toward the force producer` cylinder 62, spring operating rod E! bears against cap i8 to compress the compression spring, as the opposite ca'p- 'I9 cannot move because the cylinder is fixed to the airframe. In this case, spring operating rod 6l slides over tension rod 1l.

in the reverse direction. when force lever 5i! is moved away from the force producer cylinder 62,Y shoulder 'l5 on spring operating rod 5l picks up internal nut 'i6 on tension rod Tl, moves the tension rod l1 and opposite spring cap 19 to again compress the compression spring 8l to supply the restoring force.

The connection of force lever 60 to cam sector 57 is such that when the spring operating rod di is not compressing compression spring il! in either direction, the cam follower S6 will be at the innermost penetration in the indent portion 59, with. the force applied by the cam follower when the airplane is in flight applied radially to the cam. Thus, when the airplane is not in flight, the centering forces are applied almost entirely from the elevator force producer l, a very small amount being supplied by the diaphragm spring 'H2 which merely keeps the cam follower in the indent when the airplane is at rest. 1n flight, however, the centering forces are supplied both from the force producer 8l and from the bellows as the cam surface 58 is rotated, the latter force providing thev synthetic feel in elevation control when the stick is lmoved for that type of control.

As the stick, when moved fore-and-aft, ro`

tates the torque tubes I and 2 in directions to provide elevation control via quadrants 22 and 23, cables 25 and 26 and actuating cylinders 35 connected to the elevons, to make the elevons move in the same direction, the length of extensible link 55 should be such that when the elevator force producer and the bellows cam mechanism determine the stick position, this position for normal flight should hold the elevons in a neutral position. When out-of-trim moments occur, the pilot will instinctively move his stick to trim the airplane, and when he does so in elevation directions both the elastic and aerodynamic restoring forces are applied to the stick necessitating the continuous application of force by the pilot, with operation of the airplane from and to this trim position for maneuvering the airplane. A change in length, however, in the proper direction, of extensible link 55 will permit both centering forces to neutralize without moving the stick, thus providing a new neutral operating position for the stick without the necesu sity of application of pilot force. This new position will be one in which the control surfaces will be held in a trimming position, and trim is achieved.

One means and method of changing the length of extensible link 55 is shown in Figures 2 and 6 where the relation of the hydraulic trim cylinder 5| is changed with respect to an internal trim piston 85 attached to trim piston rod 52.

As shown in Figures 5 and 5A, the forward end of the trim cylinder 5| is closed and rigidly attached to the trim rod 49. At the rear end of the trim cylinder 5|, a cylinder head 85 is installed, this head having a rod bushing Sl through which the trim piston rod 52 slides. Trim piston rod 52 is attached to the trim piston 85 working in a iiuid chamber 88 within the trim cylinder 5i. On the opposite side of trim piston 85 from its piston rod 52 is attached a dummy piston rod 23e slidable through a casing bushing s0 in a hollow cylinder passage 9|. This dummy piston rod 89 has the same diameter as the trim piston rod 52 so that equal piston areas will be exposed to fluid pressure on each side of the trim piston 85.

Cylinder ports 92 are provided, one at each t end of the iiuid chamber 88, and bleeder valves 93 connected near the ports 92 permit the bleeding of air from both sides of the trim piston 85 to obtain a completely Huid-filled cylinder. Rod bushing seals 94 and head seal 05 prevent external huid leakage, piston seal 95 prevents fluid leakage past the trim piston 85, and casing bushing seals Si? prevent iiuid leakage to the forward end of the dummy piston rod B9.

Referring to Figure 6, flexible hydraulic fluid connections and 0| are made to the ports 52 of the hydraulic trim cylinder 5|. One of these connections |00 leads through pipe |02 to .a pilot-operated solenoid valve |03 and then past a ller pipe and check valve |04. Pipe |02 then divides, one portion |02@ leading to a restrictor |05 through a landing gear solenoid valve |05, the other portion |0217 leading directly into the restrictor |05. Within the restrictor, portions |0241 and |0219 are joined and are connected to the other end of trim cylinder 5| by connection pipe |02 to the other connection |0|. Attached to connection pipe 07 is a temperature compensator |00 to provide for expansion and contraction of the hydraulic i'luid, as is known in the art.

Within restrictor |05 a single oriice |09 is positioned in connection pipe |01 to restrict lthe flow to a desired value. Portion |021) within the restrictor before joining with portion |02a is provided with three oriiices I0, for still further flow restriction. The pilot-operated solenoid valve |03 is operated by the pilots trim switch button 2i on the stick handle 20, as shown in Figure 2 and the landing gear solenoid valve |05V is operated by a switch conveniently placed to be closed when the landing gear of the airplane is extended for landing purposes.

As the hydraulic trim cylinder 5| is floating and moves laterally when the stick is moved for roll control purposes and fore-and-aft when moved for elevation control purposes, the connections |00 and |0| are made of high pressure flexible hose to permit easy movement of the cylinder at all times.

The hydraulic trim cylinder and piston, together with the hydraulic circuits above described, is used to change the length of the extensible link 55 to achieve trim as follows:

Assume that the airplane is flying normally with the stick in normal neutra-l, the elevons in y normal elevation neutral and with the length of extensible link 55 such that the restoring forces applied to the stick by both the force producer 8| and the bellows 6l are zero. In this case, the trim piston will be at or near the central posi- ,tion within trim cylinder 5 Both solenoid valves E05 and |06 are closed, and the trim piston 85 is, therefore, held in position by a hydraulic lock, due to the incompressibility of the hydraulic fluid within the limits of applied pressures. Thus, extensible link 55 acts as a solid rod, to transmit the restoring forces to the stick. Y

When an out-of-trim moment occurs in elevation direction, the pilot instinctively moves his stick forward or aft to trim the airplane, and in doing so, the restoring forces generated by the force producer and the bellows act on the stick, thereby applying pressure to the fluid on one side or the other of the trim piston in the hydraulic trim cylinder 5|.

With the pilot holding the stick in trim position, he may then close the pilot switch button 2| opening the pilot-operated solenoid valve E03. The fluid under pressure can then flow through the three orifices ||0 and the single orice |00 into the other side of the trim piston until the pressure is equalized, thereby permitting the trim piston to move in the trim cylinder to shorten or lengthen the extensible link 55. The pressure will be equalized when the force producer and the connected bellows reach their neutral. The pilot then releases the trim switch button 2| to again lock the trim cylinder, and the stick and the surfaces controlled thereby will thereafter be held in its new position by the force mechanisms.

If the electrical system for the pilot-operated solenoid valve |03 should fail to work, provisions are made for manually opening the valve. This consists of a p-ull cable i leading from the valve |03 to an emergency valve operating handle il? located within reach of the pilot or co-pilot. The pilot-operated solenoid valve |03 is spring-loaded to the closed position and incorporating means enabling mechanical operation.

As it is desirable that a faster trim be achieved during take-oil and landing procedures, the extension of the landing gear opens the landing gear solenoid valve |05. Thereafter, when the landing gear is extended, when the pilots switch button 2| is closed, the fluid can now through all of the orices, with the single orifice |09 in parallel withlthel threeorifices` l I; 'Ilhis vallows the fluidtdequalizeithepressures-rapidly so that the length of link. 55s can be; changed rapidly during theusuallyflimited'timeinvolved in take-oir and landing..

Itlwill bel noted. that. the trim. system, as described above, is self-sufficient, i. e., needs no motor or source of fluid under pressure. The forces required for change oiV trim come solely from the pilot, the elevator force producer and the bellows. Furthermore, if the bellows should become inactive, the force producer' alone, when opposed by stick forces, can change the length oflink 55 under the pilots control.

It` isvalso to be noted that the relative position of theforce producermechanism andtrirn cylinder may be reversed without changing the invention. For example, in Figure 8. the force producer assembly in cylinder 62S is connected to the stick` yoke l2 while thetrim cylinder 5l is connected directly to the airframe. In this embodiment, trimming is accomplished by axially displacing the force producer assembly, by moving either the cylinder B2 or the operating rod di, depending upon the` end-for-end position or the force producer unit. In' both cases, relative movement of the force producer cylinder 62v and the spring operating rod di takesplace to relieve spring 8l compression, andthe operation or the trim cylinder 5l in both cases changes the overall length of the composite link to change the pilots control neutral as` desired. rEhe embodiment of Figure 8, however, does not provide for the inclusion of anaerodynamicstick force which will act in a manner asdescribed above.

What is claimed is:`

l. In an airplane having a control element connected to move a controlled device; a trim system comprising elastic centering means connected to the framework of said airplane, a link'- age connected between said control element and said elastic centering means to apply restoring forces produced by said centering means to said control element to determine a neutral control element position predicated on the length. of said linkage, said linkage being independent of the connection of said control element with said controlled device and comprising two relatively slidable telescopic-like members, and pilot-operated multiple-position locking and unlocking means operatively connected to said members to allow a change in length of said linkage in the proper direction automatically to relieve said restoring forces, whereby said control element assumesa new neutral position after unlocking and locking of said members by said pilot-operated means.

2. In combination with a control element of an airplane, a trim system comprising a spring casing, end caps sliding within said casing and restrained by said casing at the ends thereof, a spring positioned within said casing and bearing against said end caps, link means connected to said control elementand to said rend caps to move one or the other of said end caps in said casing against said spring in accordance with the direction of movement of said control element to create a neutral control element position predicated on the position of said casing in .said airplane along the axis of said casing, a longitudinally extensible linkage capable of free extension and retraction under axial force connected between said casing and the framework of said airplane and pilot-operated,"control means operatively connected to said. extensible linkage for both locking said extensible linkage as a solid uniti and unlocking said linkage, independently of: the position of said control` element, to allow axial movement of said casing in said airplane in accordance with the direction of spring-applied force on said casing when said control element is moved away from. said neutral` position, whereby said` control element is given a new neutra] position after operation of said pilot-operated means.

3. In combination with an airplane control element connected to move a controlled device, trim system comprising a spring casing, end caps sliding within said casing and restrained by said casing at the ends thereof, a spring positioned within; said casing and bearing against said end caps, linkmeans connected to said control element `and bearing against said end caps to move one or theother of said end caps against said spring in accordance with the direction of movement of said control element to develop restoring.forcesdetermining a neutral control element position predicated on the length oi said link meanssaid linkmeans being independent of the connection of said control element with said controlled device, and comprising two relatively slidable telescopic-like members forming the respective ends of said link means and pilotoperated means operatively connected to said members for both locking` together and unlocking said members, to allow a change in the length of said link means in accordance with the directionA of spring-applied force on said link means when said control element is away from said neutral position, whereby said control ele.- ment is given a new neutral position after operation of said pilot-operated means.

4. In combination with a control element of an airplane, a trim system comprising a spring casing, end caps sliding within said casing and restrained by said casing at the ends thereof, a spring positioned within said casing and bearing against said end caps, link means connected to said control element and to said end caps to move one or the other of said end caps in said casing against said spring in accordance with the direction or" movement of said control element to create a neutral control element position predicated onthe position or said casing in said airplane along the axis of said casing, and pilot-operated equilibrium-seeking means connected betweenV said casing and. the framework of said airplane to allow axial movement of said casing in said airplane in accordance with the direction of spring-applied force on said casing when said control element is moved away from said neutral' position, whereby said control element isV given a new neutral position after operation oi said equilibrium-seeking means, said equilibrium-seeking means including a hydraulic cylinder and piston assembly having hydraulic lines connected between opposite sides of said piston to form a closed circuit, a valve connected in said circuit, and means 'for opening and closing said valve at will.

`movement of Asaid control element to develop -re- `of the connection of said control element with said controlled device, and pilot-operated equilibrium-seeking means connected in said link lmeans to change the length of said link means in accordance with the direction of spring applied force on said link means when said control Aelement is moved away from said neutral position, whereby said control element is given a new Yneutral position after operation of said equilibrium seeking means, said equilibrium-seeking means including a floating hydraulic cylinder and piston assembly having hydraulic lines connected Abetween opposite sides of said piston to form a closed circuit, a valve connected in said circuit, and means for opening and closing said valve at will.

`6. In combination with a control of an airplane, an extensible link connected at one end to said control, elastic means connected between the other end of said extensible link and the structural framework of said airplane to exert restoring forces on said control toward a neutral control position determined by the length of said extensible link, said extensible link comprising a iioating hydraulic piston and cylinder assembly having hydraulic lines connected between opposite sides of said piston to form a closed circuit, a valve connected in said circuit, and means for opening and closing said valve at will, independently of the position of said airplane control, whereby any of a plurality of neutral control positions can be obtained.

7. In a control system of an airplane, a pilots control element connected to move an airplane control surface, elastic control element center- 'ing-means, a linkage connecting said centering means to said control element independently of the remainder of said control system, said link- Yage including a hydraulic cylinder and piston, a hydraulic iiuid path connecting the opposite sides of said piston, and means in said path for blocking Yor permitting flow between the opposite sides of said piston to respectively lock said linkage or permit change in length thereof.

8. Apparatus in accordance with claim 7 wherein remote control means is connected to said blocking means in said path, said remote control means leading to a location convenient to the pilot for operation thereby.

9. Apparatus in accordance with claim 7 wherein said elastic means is a spring.

10. Apparatus in accordance with claim 7 wherein said elastic means comprises force transmitting means connected to said linkage to bias said control element to a centered position dependent on the length of said linkage, when a force is applied to said transmitting means in a predetermined direction, and dynamic airflowresponsive force producing means connected to said force transmitting means, to apply force thereto in said predetermined direction varying with dynamic airfiow past said airplane.

11. Apparatus in accordance with claim 'I wherein said elastic means comprises force transmitting means connected to said linkage to bias saidcontrol element to a centered position dependent on the length of said linkage, when a force is applied to said transmitting means in a predetermined direction, dynamic airflow-responsive force producing means connected to said force transmittingmeans, to apply force there.

to in said predetermined direction varying with dynamic airiiow past said airplane, and a spring connected to said force transmitting means to apply a force thereto in addition to the airflowproduced force, whereby a centering force is applied to said control element in the absence of said dynamic airflow.

12. In a control system of an airplane, a pilots control element connected to move an airplane control surface, a trim control circuit connected between said control element and the structural framework of said airplane, said trim control circuit comprising an extensible linkage and an elastic means connected in series, said elastic means constructed to exert restoring forces on said control element toward a neutral control element position determined by the length of said extensible linkage, and said extensible linkage comprising a hydraulic piston and cylinder assembly having hydraulic lines connected between opposite sides of said piston to form a closed circuit, a valve connected in said closed circuit, and means for opening and closing said valve at will, independently of the position of said pilots control element, whereby any of a plurality of neutral control element positions can be obtained.

13. In combination with an airplane surface control, a mechanical trim control circuit connected to said surface control, said trim control circuit comprising an elastic means and an extensible linkage connected in series, said elastic means constructed to center said surface control at a neutral control position determined by the length of said extensible linkage, and said extensible linkage including a hydraulic cylinder and piston, a hydraulic fluid path connecting the opposite sides of said piston, and means in said path for blocking or permitting now between the opposite sides of said piston to respectively lock said linkage or permit change in length thereof, and operating means for said latter means operable independently of the position of said surface control, to close and open said path.

14. In an airplane having a pilots surface control element, a trim control circuit connected between said control element and the structural framework of said airplane, said trim control circuit comprising an extensible linkage and an elastic means connected in series, said elastic means constructed to exert restoring forces on said control element toward a neutral control element position determined by the length oi' said extensible linkage, and said extensible linki age comprising two relatively movable telescopiclike members forming opposite respective ends of said linkage, and variable-position locking means operatively connected to said membersV for locking and unlocking said members independently of the position of said pilots control element, to respectively prevent or permit relative movement of said two members, whereby unlocking of said members allows said control element to be moved relative to one end of said elastic means to obtain a new neutral control element position upon relocking of said members.

15. In an airplane having a pilcvts surface control element, a trim system comprising an extensible linkage and an elastic centering means connected in series between said control element and the structural framework of said airplane, saidextensible linkage comprising two relatively movable telescopic-like members forming opposite respective ends of said linkage, and remote-operated;variable-position coupling means operatively connected to said members for locking and unlocking said members, independently of the position of said pilots control element, to respectively prevent or permit relative movement of said two members, whereby unlocking of said members allows said control element to be moved relative to one end of said elastic centering means to obtain a new centered control element position upon relooking of said members.

16, In an airplane having a control element connected to move a control surface, an extensible link connected at one end to said control element, airflow-operated means connected to the other end of said extensible link to produce centering forces on said control element in accordance with the dynamic airflow produced in flight of said airplane, said centering forces determining a neutral control element position in accordance with the length of said extensible link, said extensible link comprising a floating hydraulic piston and cylinder assembly having hydraulic lines connected between opposite sides of said piston to form a closed circuit, a valve connected in said circuit, and means for opening and closing said valve at will. independently of the position of said control element, whereby any of a plurality of neutral control element positions can be obtained.

17. In a control system for an airplane, a pilots control element connected to move a control surface, a trim control circuit connected between said control element and the structural framework of the airplane, said trim control circuit comprising a coupling means and an elastic means connected in series, said coupling means -comprising two members engageable in a multiplicity of relative coupled positions, said elastic means constructed to exert restoring forces on said control element toward a neutral control element position determined by the relative coupled position of said members, and means connected to said members for freely disengaging and re-engaging said members at will, whereby any of a plurality of neutral control element positions can be obtained.

18. In an airplane having a pilots surface control element, a trim control circuit connected between said control element and the framework of the airplane, said trim control circuit comprising a coupling means and an elastic means connected in series, said coupling means comprising two members engageable in a multiplicity of relative coupled positions, said elastic means constructed to exert restoring forces on said control element toward a neutral control element position determined by the relative coupled position of said members, and means connected to said members for freely disengaging and re-engaging said members at will, whereby any of a plurality of neutral control element positions can be obtained.

19. In an airplane, a pilots control element connected to a control surface actuating system, a trim control system connected between said control element and the solid framework of the airplane, said trim system comprising elastic control element centering means, means for disconnecting said elastic centering means, whereby said centering means is free to assume its center position independently of the position of said. control element relative to said framework, said disconnecting means being reversibly operable to reconnect said centering means at whatever relative position said control element may be in.

20. Apparatus in accordance with claim 19 wherein said elastic centering means comprises a two-way spring assembly, and wherein said disconnecting and reconnecting means is connected between said control element and said spring assembly.

21. Apparatus in accordance with claim 19 wherein said elastic centering means comprises a two-way spring assembly, and wherein said disconnecting and reconnecting means is connected between said spring assembly and said frame- Work.

22. In an airplane control system for moving an attitude control surface, an irreversible motor connected to move said surface, motor control means, a pilots control element connected to said motor control means to move said surface without feedbacks therefrom to said pilots control element, elastic centering means normally connected to said pilots control element to determine the neutral position thereof corresponding to the neutral position of said attitude control system, and means for freely disconnecting and reconnecting said elastic centering means from and to said pilots control element at will, to allow automatic adjustment of said elastic centering means to a new desired neutral position.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,671,081 Miller May 22, 1928 1,976,479 Butler Oct. 9, 1934 2,001,988 Temple May 21, 1935 2,372,214 Loepsinger Mar. 27, 1945 2,373,575 Lemonier Aug'. 10, 1945 2,405,377 Sturgess et al. Aug. 6, 1946 2,445,343 Tyra July 20, 1948 2,559,047 Porter July 3, 1951 2,583,828 Gerstenberger Jan. 29, 1952 FOREIGN PATENTS Number Country Date 524,701 Germany May 12, 1931 

